842 research outputs found
A Singular Conformal Spacetime
The infinite cosmological "constant" limit of the de Sitter solutions to
Einstein's equation is studied. The corresponding spacetime is a singular,
four-dimensional cone-space, transitive under proper conformal transformations,
which constitutes a new example of maximally-symmetric spacetime. Grounded on
its geometric and thermodynamic properties, some speculations are made in
connection with the primordial universe.Comment: RevTeX4, 10 pages, 1 eps figure. Presentation changes, including a
new title; section II.E, on the thermodynamic properties of the de Sitter
horizon, completely revised. Version to be published in Journal of Geometry
and Physic
Occupation numbers in density-functional calculations
It is the intention of this paper to rigorously clarify the role of the
occupation numbers in the current practical applications of the density
functional formalism. In these calculations one has to decide how to distribute
a given, fixed number of electrons over a set of single-particle orbitals. The
conventional choice is to have orbitals below the Fermi level completely
occupied and the orbitals above the Fermi level empty. Although there is a
certain confusion in literature why this choice is superior to any others, the
general belief is that it can justified by treating the occupation numbers as
variational parameters and then applying Janak's theorem or similar reasoning.
We demonstrate that there is a serious flaw in those arguments,mainly the
kinetic energy and therefore the exchange-correlation potential are not
differentiable with respect to density for arbitrary occupation numbers. It is
rigorously shown that in the present context of the density functional
calculations there is no freedom to vary the occupation numbers. The occupation
numbers cannot be considered as variational parameters.Comment: 10 pages, Revtex, accepted for publication by Phys.Rev.
A New Finite-lattice study of the Massive Schwinger Model
A new finite lattice calculation of the low lying bound state energies in the
massive Schwinger model is presented, using a Hamiltonian lattice formulation.
The results are compared with recent analytic series calculations in the low
mass limit, and with a new higher order non-relativistic series which we
calculate for the high mass limit. The results are generally in good agreement
with these series predictions, and also with recent calculations by light cone
and related techniques
Universal energy distribution for interfaces in a random field environment
We study the energy distribution function for interfaces in a
random field environment at zero temperature by summing the leading terms in
the perturbation expansion of in powers of the disorder strength,
and by taking into account the non perturbational effects of the disorder using
the functional renormalization group. We have found that the average and the
variance of the energy for one-dimensional interface of length behave as,
, , while the distribution
function of the energy tends for large to the Gumbel distribution of the
extreme value statistics.Comment: 4 pages, 2 figures, revtex4; the distribution function of the total
and the disorder energy is include
Charge Density Wave-Assisted Tunneling Between Hall Edge States
We study the intra-planar tunneling between quantum Hall samples separated by
a quasi one-dimensional barrier, induced through the interaction of edge
degrees of freedom with the charge density waves of a Hall crystal defined in a
parallel layer. A field theory formulation is set up in terms of bosonic
(2+1)-dimensional excitations coupled to (1+1)-dimensional fermions. Parity
symmetry is broken at the quantum level by the confinement of
soliton-antisoliton pairs near the tunneling region. The usual Peierls argument
allows to estimate the critical temperature , so that for mass
corrections due to longitudinal density fluctuations disappear from the edge
spectrum. We compute the gap dependence upon the random global phase of the
pinned charge density wave, as well as the effects of a voltage bias applied
across the tunneling junction.Comment: Additional references + 1 figure + more detailed discussions. To be
published in Phys. Rev.
Open inflationary universes in a brane world cosmology
In this paper, we study a type of one-field model for open inflationary
universe models in the context of the brane world models. In the scenario of a
one-bubble universe model, we determine and characterize the existence of the
Coleman-De Lucia instanton, together with the period of inflation after
tunneling has occurred. Our results are compared to those found in the Einstein
theory of Relativistic Models.Comment: 8 pages, 4 Figures, accepted in Physical Review
Superconductivity with hard-core repulsion: BCS-Bose crossover and s-/d-wave competition
We consider fermions on a 2D lattice interacting repulsively on the same site
and attractively on the nearest neighbor sites. The model is relevant, for
instance, to study the competition between antiferromagnetism and
superconductivity in a Kondo lattice. We first solve the two-body problem to
show that in the dilute and strong coupling limit the s-wave Bose condensed
state is always the ground state. We then consider the many-body problem and
treat it at mean-field level by solving exactly the usual gap equation. This
guarantees that the superconducting wave-function correctly vanishes when the
two fermions (with antiparallel spin) sit on the same site. This fact has
important consequences on the superconducting state that are somewhat unusual.
In particular this implies a radial node-line for the gap function. When a next
neighbor hopping t' is present we find that the s-wave state may develop nodes
on the Fermi surface.Comment: 10 pages, 9 fig
Designing an Educational Game: Case Study of ’Europe 2045’
Abstract. This paper presents a theoretical framework, which has been adopted in designing an on-line multi-player strategy game Europe 2045. Europe 2045 is an educational tool for high school social science courses, aimed at familiar-izing students with political, economic, and social issues in contemporary Europe. Apart from learning facts, players develop a range of key skills: discus-sion ability, negotiation, teamwork, and group decision-making. The presented theoretical framework is based on a critical analysis of crucial issues, which seem to determine the success or failure of development and implementation of an educational game in the formal school environment. It demonstrates key ap-proaches the authors of Europe 2045 have adopted in order to overcome already known problems related to game-based learning. On a general level this paper discusses issues related to formal fact learning in educational systems and the possible role of educational games in enhancing these systems
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